1. Field of the Invention
The invention relates to a pad structure, and particularly relates to a pad structure having a staircase structure.
2. Description of Related Art
For the purposes of high density and high performance, seeking further developments in the three-dimensional space within a limited unit area has now become the trend as the semiconductor devices are integrated. Conventional three-dimensional semiconductor devices, such as NAND flash memory of the non-volatile memory, include a vertical memory array where a plurality of memory cells are arranged.
Taking the three-dimensional non-volatile memory as an example, even though the three-dimensional semiconductor devices have an increased memory capacity within the unit area, the structure of vertical memory array makes it more difficult to electrically connect memory cells at different layers with an external part. Thus, how to electrically connect the memory cells in the three-dimensional non-volatile memory with the external part while keeping the unit area minimized is an issue for researchers to work on.